This invention relates to electrodes. More specifically, this invention relates to high-temperature electrodes for use as current collectors in the channel of a magnetohydrodynamic generator.
In a magnetohydrodynamic power generator, heat is utilized to produce a high-velocity stream of electrically conducting fluid or plasma which is passed through a magnetic field to convert the kinetic energy of the stream into electrical energy. A typical diagonal window frame MHD power generator is an elongated duct or channel constructed of a large number of open rectangular frames or "window frames" fastened together side by side, insulated from each other and cooled by a liquid passing through coolant channels in each frame. Around the inner perimeter of each frame are attached, generally by brazing, a number of individual, generally rectangular, electrodes for collecting the electrical current generated in the channel by the passage of the high-temperature conductive fluid through the magnetic field. Other generator geometries can also be used, but, in each case, a number of electrodes are present on each frame mounted end to end and separated from each other by an electrical insulation since some will act as anodes and some as cathodes as the plasma passes through the channel perpendicular to the longitudinal axis of the electrodes.
The environmental conditions within an operating channel in which the electrodes must function are very severe, and strenuous physical demands are placed on these electrodes. The plasma, which may be either an ionized gas or an inert gas seeded with a conductor such as potassium, may reach temperatures up to 2800.degree. , while the surface of the electrode may reach about 2000.degree. C. However, since the window frames to which the electrodes are attached are generally of copper, the electrode-frame temperatures can be no more than about 600-1000.degree. C. Thus, the electrodes must be capable of withstanding a temperature differential between electrode-plasma interface and the electrode-frame interface of up to about 1400.degree. C. Minimizing the temperature differential within the plasma between the plasma core and the electrode-plasma interface increases the energy conversion efficiency. The electrode must be able to withstand erosive forces since the plasma as it passes through the duct may approach or even exceed sonic velocity. The electrode must either be protected from oxidation or be prepared of oxidation-resistant materials since many plasmas, depending upon the particular fluid and its source, are slightly oxidizing at operating temperatures. The electrode must also be able to withstand the effects of potassium at operating temperatures when present as seed material in the fluid. The electrode must be constructed of materials which are electrically conductive at the normal operating temperature of the electrode, which usually requires that the electrode be constructed of several different materials because of the temperature differential through the electrode. Finally, since there is always the possibility of generator malfunction, the electrodes must be able to withstand the thermal shock of sudden heating or cooling without the electrode separating from the channel or without the upper high-temperature erosion-resistant layers spalling from the remainder of the electrode. Thus, it is a problem to find a material or materials and an electrode design from which electrodes can be made which can withstand the rigor of such an environment.